It mostly just contains graphical changes, and adds optional ray tracing which I wouldn’t suggest unless you have a very powerful computer.

It had some issues when it first came out, but it seems to work fine now from my experience. Don’t expect anything groundbreaking but it’s a nice update. Textures especially look better overall.

https://www.thewitcher.com/us/en/news/47105/next-gen-update-list-of-changes

I just started playing it last night, never played any Witcher game before. It's pretty cool so far, works well on the steam deck.